The production of organic chemicals by microorganisms is well known to those familiar with the fermentation art. Such fermentation reactions frequently produce a variety of products in dilute aqueous solutions. The expense of separating the chemicals from each other and from the large volumes of water has been so great that production of chemicals by fermentation has not been able to compete with production of the same chemicals from fossil fuel sources. However, the gradual depletion of petroleum fossil fuel with the resultant increase in prices of petrochemical feedstocks has revived interest in such fermentation reactions which can convert carbohydrates that are renewable raw materials into simple organic chemicals.
Homoacidogenic fermentation reactions are of particular interest because they produce a single acidic compound in the fermentation. Products which can be obtained by these fermentations include the industrially important acetic and lactic acids. The fermentation of glucose by Clostridium thermoaceticum (hereinafter written C. thermoaceticum) is especially attractive since it can produce theoretically 3 moles of acetate from 1 mole of glucose.
Studies of these fermentation reactions have been reviewed by Zeikus, J. G., Ann. Rev. Microbiol., 34, 423-464 (1980). He has classified the microorganisms useful for carrying out chemical-producing fermentations. These have been divided into three classes: the acidogens, the solventogens and the methanogens, which produce acids, solvents and methane respectively. Among the acidogens, the homoacid-forming species that produce either acetic, lactic, or butyric acid are described as being the most interesting in terms of product yeilds.
It would be of considerable commercial interest, therefore, if a process could be developed for the production of acids using these fermentation reactions in a continuous mode. In a recent disclosure, Wang, G. Y. and Wang, D. I. C., 178 National A.C.S. Meeting, Las Vegas, Nev., August, 1980, a method for immobilizing the thermophilic anaerobic homoacidogenic bacterium, C. thermoaceticum, in agar and carrageenan gel was described. To test the stability of this gel for continuous acetic acid production, a repeated batch experiment was performed using the gel. Acetic acid productivity at the rate of 2.2 grams per liter-hour (g/1-hour) was achieved. Although this is a distinct improvement over the 0.5 g/1-hr produced by the cells in a batch fermentation, it is still unacceptably low.
In order for a continuous fermentation process to be acceptable for commercial use, it must be operated at a high dilution rate. Dilution rate is a value obtained by dividing the flow rate of the fermentation medium through the reactor by the volume of the reactor. Furthermore, the volumetric productivity, i.e., the amount of product formed per unit volume of reactor in a given time, must be high when the fermentation is carried out at a high dilution rate. A continuous fermentation process has now been discovered which can be carried out at a high dilution rate and which gives a volumetric productivity much greater than that of the best process previously disclosed.